1. Field of the Invention
The present invention relates to a plasma display panel, and more particularly to a technology for improving a bright room contrast ratio.
2. Description of the Related Art
Plasma display panels (hereinafter, also referred to as PDPs) are display panels of self-luminous type, and are receiving attention as display panels that replace CRTs (Cathode Ray Tubes) by virtue of their high visibility and low profiles. A PDP is formed by filling discharge gas into a space of the order of 100 microns sandwiched between two glass substrates (a front substrate 26 and a rear substrate 34 in FIG. 2 to be described later) which are provided with electrodes. One of the glass substrate is coated with phosphors. Then, a voltage higher than or equal to a starting voltage is applied between the electrodes to cause a discharge, and the ultraviolet rays generated from the discharge make the phosphors excitation-luminous for pixel luminescence.
FIG. 1 shows an overview of one PDP 10 called a surface-discharge alternating-current type, among PDPs of this kind.
The PDP 10 is provided with a plurality of pairs of discharge electrodes 12 and 14 which extend in the horizontal direction of the diagram, and a plurality of address electrodes which are orthogonal to these discharge electrodes 12 and 14. The discharge electrodes 12 and 14 include transparent electrodes 18 and nontransparent bus electrodes 20 formed on these transparent electrodes 18. The transparent electrodes 18 are formed of tin oxide (SnO2) or ITO (a transparent conductor consisting mainly of indium oxide), and have a relatively high resistance. The bus electrodes 20 are formed of metal such as copper. These bus electrodes 20 lower the resistances of the discharge electrodes 12 and 14.
Besides, a pair of discharge electrodes 12 and 14 form a display line L. A predetermined gap (non-display area) is arranged between neighboring display lines L so that the discharge electrodes 12 and 14 will not cause any accidental discharge across the two lines. In order to avoid a drop in bright room contrast ratio due to external light reflection, a black stripe 22 is formed in this gap.
Ribs 24 are formed between and along these address electrodes 16. Then, the regions surrounded by the black stripes 20 and the ribs 24 form cells C, or light emission units.
As shown in FIG. 2, the discharge electrodes 12, 14 and the black stripes 22 are formed on the inner, or interior, surface, adjacent the discharge space 28, of the front substrate 26, the exterior surface of which is a display surface for an observer. A dielectric layer 30 for holding a wall charge and a protection layer 32 made of magnesium oxide (MgO) are formed over the discharge electrodes 12, 14 and the black stripes 22.
Meanwhile, as shown in FIG. 3, the address electrodes 16 and the ribs 24 are formed on an inner, or interior, surface, adjacent the discharge space 28, of the rear substrate 34. A dielectric layer 36 is formed over the address electrodes 16. The ribs 24 are formed on this dielectric layer 36. Phosphor layers R, G, and B are formed over the inclined planes of the ribs 24 and the dielectric layer 36 surrounded by the ribs 24. The phosphor layers R, G, and B respectively emit red light, green light, and blue light, by the incidence of discharge-generated ultraviolet rays. That is, in this example, a single pixel capable of full color display is composed of three cells.
In the above-described PDP, before pixel display, a reset pulse is applied to (i.e., across) the discharge electrodes 12 and 14 to initialize the cells (reset period). Then, address pulses are applied to address electrodes 16 that correspond to data to be displayed, thereby selecting cells C to emit light (address period). Then, sustain pulses are applied to (i.e., across) the discharge electrodes 12 and 14 over periods corresponding to the brightness gradations, to sustain discharges in the selected cells C (sustentation, or sustain, period). Ultraviolet rays generated from the sustain-discharge excite the phosphor layer R (or G, B) to emit light. Then, the light is transmitted through the transparent electrodes 18 and the front substrate 26 to radiate out to the exterior, thereby displaying an image.
FIG. 4 shows an overview of another PDP 38 disclosed in Japanese Patent No. 2801893 Gazette. This kind of PDP is referred to as ALIS (Alternate Lighting of Surfaces) technology.
The PDP 38 has a plurality of discharge electrodes 40 formed at regular intervals. Address electrodes 16 and ribs 24 are arranged as in FIG. 1. The black stripes 22 shown in FIG. 1 are not formed in this PDP 38. On this account, the discharge electrodes 40, except the electrodes 40 at opposite ends, or edges, can produce discharges, with their respective adjacent discharge electrodes 40 on both sides. That is, cells C, or light emission units, are formed to overlap with each other along the address electrodes 16. Display lines L are also formed to overlap with each other. As a result, given an equal definition (i.e., an equal number of lines L), the number of discharge electrodes is only about half that in the PDP 10 of FIG. 1. The absence of non-luminescence regions allows an improvement in brightness if the panel sizes are identical.
FIG. 5 shows a cross section of the PDP 38 taken along an address signal 16, and luminescent intensities along the cross section.
In the luminescent intensity (1), the solid line indicates the intensity for situations where the display line L1 emits light, and the broken line indicates the intensity for situations where the display line L2 emits light. More specifically, the luminescent intensity on each line reaches the maximum in the middle of the neighboring discharge electrodes 40, and decreases with distance from the middle. The display lines L1 and L2 repeat alternate luminescence successively. Therefore, the actual intensity distribution, as shown in the luminescent intensity (2), is given by the sum of the solid line and the broken line in the luminescent intensity (1). Accordingly, the entire PDP 38 offers the maximum luminescent intensity in the very middles of the spaces between discharge electrodes 40.
FIG. 6 shows a cross section of the PDP 38 taken along a discharge electrode, and luminescent intensities along the cross section.
The solid line indicates the luminescent intensity for situations where the ribs 24 are formed of nontransparent material, and the broken line indicates the luminescent intensity for situations where the ribs 24 are formed of a transparent dielectric or the like. The luminescent intensities have three peaks. Of these, one lies in the portion where the address electrode 16 and the discharge electrode 40 face each other, while the other two fall on the inclined planes of the ribs 24. The facing portion of the address electrode 16 and the discharge electrode 40 is where the discharge becomes the most active; a large amount of ultraviolet rays occur for higher luminescent intensity. The inclined planes of the ribs 24 increase in radiation density as seen from the side of the front substrate 26. On the inclined planes, the substantial radiations from the phosphor layer R (or G, B) strengthen each other to make the luminescent intensity higher than in the central part of the cell C.
By the way, the PDP 38 of ALIS technology shown in FIG. 4 improves in brightness as compared with the PDP 10 shown in FIG. 1, whereas it has a higher surface reflectance ratio because of having no non-luminescence regions other than the ribs 24 and the bus electrodes 20. Specifically, while the PDP 10 having the black stripes 22 shown in FIG. 1 is lower than or equal to 20% in surface reflectance ratio, the PDP 38 of ALIS technology shown in FIG. 4 reaches 30–40% in surface reflectance ratio. Consequently, the PDP 38 of ALIS technology had a problem that the external light reflection increases to lower the bright room contrast ratio.
If the bright room contrast ratio drops, the screen of the PDP 38 looks whitish all over in bright rooms. In general, PDPs are provided with an optical filter at their front to decrease the transmittance for the sake of higher bright room contrast ratios. Simply arranging an optical filter at the front, however, lowers the brightness of the entire screen.